Anaerobic digesters help to stabilize sewage sludge and biosolids in wastewater treatment plants before it is either used in agricultural land or dried, incinerated or landfilled.
The product of anaerobic digestion, which is commonly called biogas, is a mixture primarily composed of methane and carbon dioxide, and usually undesired substances such as water, hydrogen sulphide, ammonia, oxygen, siloxanes and particulates.
Hydrogen Sulphide pre-cleaning of biogas is necessary to be used as fuel for combined heat and power (CHP) units or boilers and further cleaning and upgrading is required to be used as fuel for vehicles or gas grid injection.
Biogas upgrading refers to removal of carbon dioxide from biogas. The energy content of biogas is in direct proportion to the methane concentration, in such a way that the energy content of the gas is increased by removing carbon dioxide in the upgrading process, becoming comparable to natural gas.
There are four main techniques for upgrading biogas to vehicle fuel quality, among others: Absorption by water scrubbing (which is considered to be the closest prior art), PSA (Pressure Swing Adsorption), organic physical scrubbing and absorption with chemical reaction or membranes separation.
Biogas cleaning refers to the removal of the cited undesired substances from biogas, apart from carbon dioxide. The unwanted substances can be removed before, during or after the upgrading stage.
Pre-cleaning should be understood as removing the unwanted substance, mainly hydrogen sulphide, before the upgrading step in order to prevent corrosion and mechanical wear of the upgrading equipment itself. In addition, hydrogen sulphide can cause problems during the removal of carbon dioxide and it involves problems of bad odors.
There are many chemical, physical, and biological methods currently available for removal of hydrogen sulphide from biogas. Dry based chemical processes have been traditionally used for biogas applications, i.e. Iron Sponge and potassium-hydroxide-impregnated-activated-carbon systems are the most desirable ones. These processes are simple and effective, but also incur relatively high labor costs in order to handle and dispose of materials. Other significant drawbacks include a continually produced stream of solid waste and a growing environmental concern about appropriate disposal methods.
Addition of air by injection (2-6%) to the digester bed or headspace, or iron compounds introduced directly into the digester, have shown promising results as the partial removal method of H2S. However, these methods show limited and inconsistent operating performances. Furthermore, oxygen is limited for biogas as vehicle fuel (<1% volume, according to the Swedish Standard) and it should be avoided. Liquid based and membrane processes require significantly higher capital, energy and media costs, and do not appear as economically competitive as selective H2S removal. Commercial biological processes for H2S removal that boast reduced operating, chemical, and energy costs are available, but they require higher capital costs of installation than dry based processes. Therefore, a low capital and operating cost for H2S removal from biogas is a need to address for biogas upgrading within current available technologies.                Upgrading process itself: In upgrading technologies where carbon dioxide is separated from biogas, some of the other unwanted compounds are also separated during the upgrading process itself.        Final polishing step: The biogas has to be dried and purified of hydrogen sulphide and siloxanes to obtain vehicle fuel quality, e.g. total sulphur <20 mg/Nm3 and Water dew point of −10° C. at 200 bar, according to ISO 6327. Drying and purification can be obtained by cooling and adsorption using i.e. SiO2, activated carbon or molecular sieves as in U.S. Pat. No. 8,747,522. These materials are usually regenerated by i.e. heating or a decrease in pressure. Other technologies for water removal are absorption in glycol solutions or the use of hygroscopic salts.        
Cleaning and upgrading involves a notable cost in the biomethane production process. It is therefore important to have an optimized cleaning and upgrading process in terms of energy, water and chemical consumption and high purity methane as a final product.
Full Scale Technologies for Biogas Upgrading
When biogas is used as fuel for vehicles or gas grid injection it has to be upgraded, cleaned and compressed. A number of technologies have been developed to that end: absorption and desorption (PSA) methods.
1. Absorption
In an upgrading plant using the absorption technique the raw biogas meets a counter flow of liquid in a column which is filled with plastic packing (in order to increase the contact area between the gas and the liquid phase). The principal behind the absorption technique is that carbon dioxide is more soluble than methane, in such a way that the liquid leaving the column will thus contain an increased concentration of carbon dioxide, whereas the gas leaving the column will have an increased concentration of methane. Three examples of the absorption technology using different types of absorbents are water scrubbing, organic physical scrubbing and chemical scrubbing. Water scrubbing is the most common upgrading technique and plants are commercially available from several suppliers in a broad range of capacities.
1.1. Water Scrubbing
It is considered to be the prior art closest to the present patent.
1.1.1. Theoretical Background
To understand the present patent it is necessary to explain the basic principles of the process. Water scrubbing is used to remove carbon dioxide. It can also be used for hydrogen sulphide removal, but only if H2S<500 mg/m3 due to equipment corrosion and packing clogging problems. Removal of ammonia also occurs since these gases are more soluble in water than methane. The absorption process is purely physical.
The rate of gas mass transfer, i.e. from carbon dioxide to the liquid phase (water) is subject to the terms described in formula (1):
                                          ⅆ            C                                ⅆ            t                          =                                            K              La                        ×            driving            ⁢                                                  ⁢            force                    =                                    K              La                        ×                          (                                                C                  Sat                                -                C                            )                                                          (        1        )            whereinC=Dissolved gas (i.e. carbon dioxide) concentration in liquid (mass or mols/volume).Csat=Dissolved gas (i.e. carbon dioxide) at saturation in liquid (mass or mols/volume).KLa=Overall mass transfer coefficient of gas (i.e. carbon dioxide) at temperature of absorption (1/time).(Csat−C)=driving force of the process. Csat, saturation concentration of the gas, it is calculated using Henry's Law and varies with temperature (effect on KH) and Pgas, the partial pressure of the gas, according to (2)Csat=KH×Pgas  (2)
Henry's constant at 25° C. (KH) for hydrogen sulphide is 1.0×10−1 M/atm, carbon dioxide is 3.4×10−2 M/atm and for methane 1.3×10−3 M/atm (Stumm & Morgan 1996), resulting in a solubility for hydrogen sulphide that is approximately 3 times higher than for carbon dioxide, and for carbon dioxide that is approximately 26 times higher than for methane. If the raw biogas consists of 50% methane and carbon dioxide respectively, the partial pressures of these gases will be equal in the bottom of the absorption column. Furthermore, if 100% of the carbon dioxide is dissolved in the water, at least 4% of the methane will also be dissolved in the water in an ideal system.
From formulae (1) and (2) it can be concluded that gas solubility in water scrubbing increases with the following strategies:                Strategy 1: Increasing KLa, gas-water contact efficiency. In practice by the number of plates (height of column) and optimizing type of packing.        Strategy 2: Increasing the driving force, (Csat−C), which can be done by:                    Strategy 2.1. Increasing Csat when reducing the temperature (effect on KH) and increasing Pgas.            Strategy 2.2. Decreasing C which is possible by using a large volume of water for absorption, that is to say, increasing the influent liquid-to-gas ratio (LI/G).                        
Current water scrubbing systems use the lowest possible influent LI/G ratio in order to minimize water consumption, as in patent WO 2008/116878, with LI/G values of 0.1 to 0.3 m3 water at 20° C./Nm3 biogas per hour. The lowest LI/G is expressed as Lmin/G, where Lmin is the equivalent water flow rate to dissolve the carbon dioxide until a saturation equilibrium is reached, and C=Csat resulting at the outlet of the absorption column. In those systems average C in the absorption column is high, and that is the reason why strategy 2.2 is not used. Instead, strategies 1 and/or 2.1 are used.
1.1.2. Description of the Process
The biogas is brought into contact with wash water at a high pressure; components of the biogas, mainly CO2, are absorbed into the water until the saturation equilibrium is reached. Then, gas absorption ends since the LI/G equilibrium has been reached.
The raw biogas from the digester is just above atmospheric pressure and the water is saturated. Moisture and particles are removed at the inlet separator, then the gas is compressed up to 7-14 bars the biogas intake temperature can be from 15 to 38° C. (eg. 30° C.) and an outlet temperature after compression can be 70° C. for a discharge of 9 bars. Raw gas entered the absorption vessel at the bottom whereas water is fed at the top thereof and so the absorption process is operated in counter-current. The absorption vessel is provided with random packing in order to obtain maximum mass transfer. In this type of vessel, carbon dioxide is absorbed by the water and the biogas which leaves the vessel is enriched with methane. The gas leaving the absorption vessel has a methane concentration from 70 to 98% by volume, depending initial biogas composition and quality required. Before the upgraded gas is transported to the storage tank, it passes to a final polishing step, as described above to remove water to control the dew point below −80° C. and small amounts of hydrogen sulphide, as in WO 2009/116868.
Finally, the upgraded gas is odorized in order to be able to detect gas leakages, should they occur.
Since methane is partly water soluble, the water from the absorption vessel is conveyed to a flashing vessel in order to lower the methane losses. The water is de-pressurized in the flashing vessel down to 2 bars and the dissolved gas comes out. The dissolved gas, which contains some methane but mainly carbon dioxide, is released and transferred back to the raw gas inlet.
—Water Scrubbing with Regeneration
The water having the absorbed carbon dioxide and/or hydrogen sulphide which is contained in and leave the flashing vessel can be regenerated and recirculated back to the absorption column. The regeneration is carried out by air stripping in a desorption vessel, which is similar to the packed absorption columns to obtain a large mass transfer efficiency. The regenerated water is heated mainly by means of the energy input of the recycling pump; hence, it must be cooled before it is returned to the absorption vessels. High efficiency is obtained at less than 7° C. by the water chilling process. A lower process temperature results in reduced system pumping costs, hence the total energy consumption of a plant with water chilling is lower.
The stripping air contains CO2 and H2S gases and needs to be treated by using an odor control process in order to avoid any nuisance before discharging air to the atmosphere. Besides, off-gases contain a methane concentration of 1-2%. It is important to minimize the loss of methane in order to achieve an economically viable upgrading plant. It is also important to minimize the methane slip since methane is a strong greenhouse gas. Methane can be present in the off-gas leaving a PSA-column or water scrubber with water recirculation or in water in a water scrubber without water recirculation. Thus, the release of methane to the atmosphere should be minimized by treating the off-gas or the water streams coming out of the plant even though methane cannot be used.
The off-gas from an upgrading plant is extremely difficult to treat because it seldom contains a high enough concentration of methane to maintain a flame without the addition of natural gas or biogas (energy consumption). One way of limiting the methane slip is to mix the off-gas with air that is used for combustion. Alternatively the methane can be oxidized by thermal or catalytic oxidation if the methane content is above 3%.
The treatment of the off-gas containing even less methane is increasingly difficult. Since not enough energy is provided during the combustion of this gas and raw biogas, biomethane has to be added in order to reach a stable oxidation, therefore reducing the overall energy balance and economy of the system. In the regeneration option, tap water is used. In spite of this, clogging of packings can occur. This is due to bacteria and other biological material entering the plant through the air that is added to the desorption column in order to drive out the carbon dioxide from the water. This means that the packings must be removed and washed by hand.
In addition, when biogas has a hydrogen sulphide content, the recycled water will soon be polluted soon with elementary sulphur which causes operating problems.
Therefore, the regeneration of water lowers the water usage but increases the energy and maintenance consumption.
Several patents have addressed this technology: US 2010/0107872 A1, WO 2009/116878 A1, US 2008/01344754 A1, and WO 2012/128648 A1.
—Water Scrubbing without Regeneration
In the other type of absorption, water is not regenerated in a desorption column. Instead of this, it is led way from the plant after the flash tank. This is more cost effective than regenerating the water if inexpensive water, such as treated sewage water, can be used. Since water is not regenerated there, no problem occurs with precipitation of elementary sulphur in the packing of the stripping vessel. Methane which is dissolved in water and not separated in the flash tank leaves the plant with the sewage water, thus the methane slip and the sulphide odors can be a problem in the final water discharge.
Clogging or biological growth on packings in the absorption column is an existing problem in upgrading plants without regeneration of water, when using treated sewage water as the water source. In those cases there are some biological materials that get stuck in the packings or cause growth.
1.2. Chemical Scrubbing
Chemical scrubbers use amine solutions. Carbon dioxide is not only absorbed in the liquid, but also reacts chemically with the amine in the liquid. Since the chemical reaction is strongly selective, the methane loss might be as low as <0.1%, and thus, no further off-gas treatment to reduce the methane emissions is necessary (U.S. Pat. No. 8,500,864 B2, 2013; and WO2011/136733 A1).
The application of this technology is advantageous if high methane recovery is desired. A drawback is the high heat demand of the regeneration step at 160° C. Besides, the projected plant capacity is medium to large. It is not feasible for small plants.
1.3. Organic Physical Scrubbing
Organic physical scrubbing is very similar to water scrubbing, with the important difference that the carbon dioxide is absorbed in an organic solvent such as polyethylene glycol. Carbon dioxide is more soluble in polyethylene glycol than in water. Therefore, there is less demand for recirculation of the solvent and the pumping costs are lower. However, the polyethylene glycol solution is regenerated by heating and/or depressurizing and thus energy costs are higher than in other technologies. Hydrogen sulphide, water, oxygen and nitrogen may be removed together with carbon dioxide. However, more often a previous step of H2S cleaning is required.
2. Pressure Swing Adsorption (PSA)
With this technique, carbon dioxide is separated from the biogas by adsorption on a surface under high pressure. The adsorbing material, usually activated carbon or zeolites, is regenerated by a sequential decrease in pressure before the column is reloaded again, hence the name of the technique. An upgrading plant, using this technique, has four, six or nine vessels working in parallel. When the adsorbing material in one vessel becomes saturated the raw gas flow is switched to another vessel in which the adsorbing material has been regenerated (pressure-swing method). During regeneration the pressure is decreased in several steps. The gas that is desorbed during the first and eventually the second pressure drop may be returned to the inlet of the raw gas, since it will contain some methane that was adsorbed together with carbon dioxide. The gas desorbed in the following pressure reduction step is either conveyed to the next column, or it is released to the atmosphere if it is almost entirely methane free.
If hydrogen sulphide is present in the raw gas, it will be irreversibly adsorbed by the adsorbing material. In addition, the water present in the raw gas can destroy the structure of the material. Therefore hydrogen sulphide and water need to be removed before the PSA-column.
Several patents have addressed this technology:                Adsorption with zeolites (WO09/5876 or WO2008/072215)        PSA with vacuum regeneration (U.S. Pat. No. 5,797,979).        
The main drawbacks of this technology are:                Irreversible porous media saturation with a pollutant such as hydrogen sulphide, preventing the regeneration thereof.        Disposal of media at the end of the useful life        Methane can be present in the off-gas leaving a PSA-column. The methane slipped to the atmosphere is higher than those produced by the water scrubbing.        Complex design with 4 to 9 parallel vessels, multiple compressors, pipes and valves.        High operating cost, as it is an energy intensive method for adsorption and regeneration.        